In recent years, a Pb-free solder which does not contain a lead component is beginning to be used as a substitute for a Sn—Pb-based solder in consideration of environmental problems. As a solder material to be applied to a semiconductor device such as an IGBT module (power module), a Sn—Ag-based Pb-free solder, which has relatively good balance, particularly in terms of bondability (solder wettability), mechanical property, thermal resistance, and the like and which is actually employed in products, is widely used among presently known lead-free solders of various compositions.
In a semiconductor device having a layered connection structure in which an insulating substrate is mounted on a heat sink and a semiconductor element is further solder-joined thereon, there is known a solder joining structure in which a lower joining employs a Sn—Sb-based solder as a high-temperature lead-free solder, and an upper joining employs a lead-free solder with a composition of a Sn—Ag-based solder, having a melting point lower than the Sn—Sb-based solder, added with an element such as Cu (for example, see Patent Document 1).
In addition, there is known a structure intended to prevent concentration of heat generation density by solder-joining a lead frame, which is a wiring member and also serves as a heat spreader, to a top electrode of a semiconductor element (IGBT) solder-mounted on an insulating substrate and by guiding heat generated from the semiconductor element to the lead frame (for example, see Patent Document 2).
As a solder material effective in preventing cracks at high temperatures attributed to heat generation of a semiconductor element, there is also known a tape- or wire-shaped solder material with a Sn—Sb—Ag composition which has an excellent ductility at a temperature of 170° C. and which has an excellent cold workability (for example, see Patent Document 3).